Direct-conversion counting X-ray detectors are used in X-ray imaging, for example in computed tomography, angiography or radiography. The X-ray radiation or the photons can be converted into electrical signal pulses by a suitable sensor. Counting X-ray detectors not only allow counting of events but also facilitate information about the energy of the detected X-ray quant. This creates new opportunities in medical imaging for analysis and evaluation of the signal pulses.
Materials that can be used for the sensor as the converter material for the converter element are, for example, CdTe, CZT, CdZnTeSe, CdTeSe, CdMnTe, InP, TlBr2, HgI2, GaAs or other materials. The energy of the incident ionizing radiation is converted directly into electrical charges known as electron-hole pairs. A high voltage, for instance a voltage in the range of −500 to −2000V for CdTe, CZT, CdZnTeSe, CdTeSe or CdMnTe, is applied to the converter element between one electrode as the cathode and another electrode as the anode in order to separate the charges of the electron-hole pairs produced in the converter element. The cathode can be in the form of a continuous electrode. The anode can be in the form of a pixelated electrode. The high voltage is applied to the electrode by way of an external high-voltage source via an electrically conducting contact. X-ray quanta can produce electron-hole pairs in the converter element by energy deposition. The electron-hole pairs are separated by the applied high voltage, and the charge carriers, which are selected by the polarity of the high voltage, can be extracted or allowed to drift to the anode. It is thereby possible to produce an electrical signal pulse in the readout unit and/or the evaluation unit. The converter element is typically joined face-to-face in a stacked arrangement to a readout unit and/or an evaluation unit, for example to an integrated circuit (Application Specific Integrated Circuit, ASIC), via solder joints, electrically conductive adhesives or other techniques. The electrical signal pulses are evaluated by an evaluation unit, for instance an ASIC. The stacked arrangement comprising the converter element and the readout and/or evaluation unit is joined to a further substrate, for instance a printed circuit board, a ceramic substrate such as HTCC or LTCC, for example, or others. The electrical connections for the readout of the readout unit and/or the evaluation unit can be formed by way of through-silicon vias (TSV) or wire bonding.
In counting X-ray detectors, the detection characteristics can be stabilized over time and made spatially uniform by exposing the detector material continuously to an additional illumination. As happens with exposure to X-ray radiation, the additional illumination creates electron-hole pairs in the detector material which can be extracted by way of the high voltage. The higher the luminous energy incident on the detector from the additional illumination, the more electron-hole pairs are generated and the higher is a resultant direct current component, known as the sensor bias current (SBC). The luminous energy of the additional illumination and the uniformity of the illumination of the converter element can modify the drift behavior of the detector device.
In image acquisition, unacceptable image artifacts can arise due to undesirable polarization effects. The publication “Homogene Sensorausleuchtung mit Lichtleiter” (“Homogenous sensor illumination using light guide”) by P. Sievers et al., 2014, Prior Art Journal, DOI 10.4421/PAPDEOTT003771 discloses that polarization effects can be curbed by an additional illumination, for example in the visible or infrared region. By using a detector having a diffuser for coupling-in light at the sensor surface it is possible to achieve a more uniform light-intensity distribution compared with LED circuit boards, which can be installed between anti-scatter grid and sensor.
Document US 2002/0109091 A1 discloses an X-ray detector comprising at least one converter unit for converting absorbed X-ray quanta into electrical charge signals, at least one evaluation unit for amplifying and further processing of the charge signals, and at least one data processing unit for the acquisition, further processing and output of the data. The charge signals are first amplified by an input amplifier in the evaluation unit, after which they are evaluated in parallel in a counting channel and in an integrating channel.
The publication “CIX—A Detector for Spectral Enhanced X-ray Imaging by Simultaneous Counting and Integrating” by H. Krüger et al., 2008, SPIE Medical Imaging Conference, San Diego, DOI 10.1117/12.771706 discloses a hybrid pixel detector, the design of which is based on simultaneous charge integration and photon counting.
Detector devices in computed tomography machines are constructed from a multiplicity of X-ray detectors or detector modules. For instance, some 20 to 50 detector modules can be arranged in a detector device. The detector modules are mounted in the radial direction, separated by the narrowest possible gap along the rotational axis. Each of these detector modules is illuminated separately in this arrangement by the additional illumination. As a result, the additional illumination from adjacent detector modules may be incident on adjacent converter elements. This can affect the signal stability of the detector modules. In order to avoid image artifacts, it is desirable that all the detector modules behave in the same manner. The inventors have found that it is desirable for the direct current component produced by the additional illumination to be the same in all the modules. It is desirable to adjust the illumination of the detector modules in such a way that the direct current component induced in the converter element is the same for each detector module. In practice, however, this is not the case because of manufacturing tolerances, for instance of the diffuser, potentially resulting in different module characteristics. The aim is to achieve uniform additional illumination over the entire detector device and to adjust the additional illumination accordingly.